When, where to use displacement ventilation

Displacement ventilation can be used instead of mixed-flow ventilation in HVAC systems. To evaluate its suitability, consider IAQ, comfort, energy consumption, and cost.

Raymond W. Schultz, PE, Cannon Design

03/20/2012

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When a growing institution needs a new building but has limited space, an underground facility can be an elegant design solution. However, such facilities pose a number of HVAC and air quality challenges. And when the facility is an athletic complex, the challenges become particularly interesting.

In contrast to traditional mixed-flow ventilation systems that aim to uniformly dilute the entire volume of air in a space, displacement ventilation (DV) relies on a stratification concept to consolidate heat for direct removal at the top of a space. DV supplies air at low velocities from numerous outlets close to the floor. The cool supply air flooding the floor rises as it is heated by internal sources. As the heat sources within the room lift the air, it flushes through the occupied zone and is consolidated at the top of the space for extraction. The air does not re-enter the occupied space.

As a technology, DV has been maturing through common practice in industry and in Europe, and it is now applied consistently for classrooms and offices throughout the United States. Design criteria can be referenced in ASHRAE’s System Performance Evaluation and Design Guidelines for Displacement Ventilation. ASHRAE Standard 55 Thermal Environmental Conditions for Human Occupancy can be applied to ensure that temperature, velocity, and vertical temperature gradient in an occupied zone are acceptable. ASHRAE 62.1 Ventilation for Acceptable Indoor Air Quality describes air distribution effectiveness for DV systems.

The choice to employ DV instead of conventional mixed-flow ventilation should be a team decision, with the owner, engineer, and architect all on board. Each approach has strengths and weaknesses. To evaluate DV’s suitability, consider indoor air quality, comfort, energy consumption, and cost. DV is preferred where contaminants are warmer and lighter than room air, supply air is cooler than room air, disturbances to room air are weak, ceiling heights are 9 ft or more, and low noise levels are desired.

Supplying air near the floor level rather than at the ceiling has significant design implications. To avoid creating uncomfortable drafts for occupants situated near the diffusers, DV supplies air at higher temperatures—above 63 F—and at a lower velocity. To achieve adequate cooling under these conditions, the air diffusers for DV must be much larger than those of conventional systems. These larger diffusers must be accommodated in the building layout.

The lower velocity of air delivery reduces pressure requirements and allows fans to run more slowly, consuming less energy and producing less noise. This makes DV an excellent choice when lower noise levels are desired. Also, because the supply air temperature is higher than a traditional mixed-flow air system, the economizer cycle is used well into the summer season.

Designing for variations

When applying DV, consider the ventilation rate and cooling load density to ensure that the cooling requirements align with the supply air system’s ability to offset the heat gain. In spaces where the internal heat gain density exceeds 15 Btuh/sq ft, take extra care to ensure comfort. DV can be supplemented with localized cooling terminals such as chilled cooling panels to address higher heat gain densities. Emissions from equipment or food preparation should be captured at their source and removed, and care must be taken to limit downdrafts driven by cold walls or cooled ceiling panels, which may lead to higher contaminant concentrations.

A noticeable vertical temperature gradient sets up between the floor and the ceiling. This vertically stratified temperature field needs to be stable for DV to function properly. The temperature in the space is expected to vary linearly with space height in the stratified zone and is nearly constant in horizontal directions, except for regions near diffusers. However, it is difficult to predict the temperature gradient because some of the influential parameters are difficult to account for. These parameters include the radiation between the ceiling and the floor, the variable heat output of equipment and people, and the location of people within the space. A small temperature difference between wall and room air can result in noticeable downdrafts with cold walls and updrafts with warm walls. As the load in the space varies, the buoyancy forces that drive airflow will also vary. Computational fluid dynamics analysis can be used to model the complex air and temperature patterns and predict temperature gradients in the occupied zone.

Because drafts and wide temperature stratification may reduce occupant comfort, the air temperature near the floor and the vertical temperature gradient in occupied zones are the most important parameters in evaluating DV. To reduce the temperature gradient, the supply flow rate must be increased, but this could lead to high air velocity at floor level and high draft risk, as well as consume more energy. To offset heating loads in the winter and augment vertical airflow, perimeter heating terminals such as radiant panels or fin tube can be used. Convective heating terminals can boost the buoyancy-driven airflow within the space.

When heat removal is the main objective, a temperature-based design can be used to match the supply airflow rate and temperature to the current load in the space. When contaminants are a major concern, a shift zone method is used. The shift zone is the boundary between the lower, non-recirculating zone and the upper zone, which has recirculation flow. The concentration of contaminants is at its maximum at the shift zone. The shift zone height is the height above the floor at which the total amount of air carried in convective plumes above a heat source is equal to the supply airflow distributed through diffusers. The shift zone approach is popularly applied in industry to keep the human breathing zone free of contaminants.

Relative humidity (RH) is also critical. RH above 70% is too high for comfort and may result in fungal growth. At 74 F, RH should be maintained at 50% to 60%. To accomplish this, some or all of the supply air must be cooled below room dew point. In humid climates, return air bypass improves dehumidification performance by routing outside air and some return air through the cooling coil where moisture is removed from the airstream. This cold and dry air is then mixed with the conditioned return air to produce a warmer supply air temperature with lower moisture content.

Equipment needs

Air handling unit discharge temperatures are higher for DV than for mixed flow. DV works best with a centralized plant using air handling units and chilled water designed to provide 62 to 67 F supply air to the space. Using this temperature range, free cooling may be available most of the year, and the overcooling and reheat associated with mixed-flow systems is eliminated. The actual energy consumption will depend on the control strategies and air handling systems used. Enthalpy recovery devices can be applied to reduce the dehumidification load of the cooling process.

DV adds complexity to supply air ducting because of the integration of low supply air devices within the programmed space. The architectural design needs to incorporate these large air devices. Generally, supply air can be released from wall ducts that run under windows. As an alternative, air can be supplied through floor plenums, or ceiling jets can be used to send a vertical column of air to the floor and provide personal temperature control. Offices wider than 20 ft may need more than one supply air device. Low sidewall floor diffusers integrated into column enclosures can be effective for large open-plan spaces. Diffuser location and coverage needs to be researched and carefully planned.

Fewer diffusers and less ductwork can be expected, with the understanding that DV diffusers are more expensive than conventional mixed-flow diffusers. Select supply air diffusers with a discharge velocity lower than 50 fpm, as lower supply velocities reduce drafts and noise. DV’s limited cooling capacity restricts usage to applications with relatively low heat gain. It may be found that DV is ideal for core zones but not appropriate for perimeter zones because the cooling load is too high. If DV is used in perimeter spaces, a separate heating system may be needed to maintain airflow patterns. Convectors, baseboard, and radiant panels are effective but come with additional cost.

Diffuser type is key to providing the higher air volume rates needed for higher heat loads. Displacement outlets may be located almost anywhere in a room, but the conventional approach has been to locate them at or near floor level. They should be located to take advantage of naturally occurring thermal stratification within the room. Volume control dampers can be located in branch ducts supplying diffusers.

To avoid drafts near diffusers, diffuser performance is critical, and the velocity in the occupied zone, especially near diffusers, must be well controlled, with a velocity no higher than 50 fpm. The entrainment of room air will decrease the temperature gradient in the occupied zone. Blending supply air quickly will reduce drafts. Information on diffuser aspiration and modulation can be found in product catalogs. For diffusers, the distance from a wall-mounted diffuser to a 40 fpm velocity contour along the center line is an important parameter for comfort.

Flow from several diffusers placed close together on the wall will merge to a two-dimensional flow, resulting in velocity lower than a radial-flow single diffuser. If diffusers have slanting discharges and are too close together, the supply airflows can meet and project straight into the room for several feet.

Applications

Typically, ceilings should be higher than 9 ft for the displacement effect to take place. Higher ceiling heights enable DV to remove larger heat loads. DV should not be used if high temperatures above 7 ft are unacceptable. It blends well with vaulted ceilings, daylighting, and radiant floor heating. DV may also be useful in core spaces where cooling is always needed. However, perimeter zones with high cooling loads need a close look. The supply airflow rate should be the higher of the required ventilation airflow rate and the supply airflow rate based on cooling load.

Compliance with ASHRAE 55 ensures that the supply temperature, velocity, and vertical temperature gradient in the occupied zone are acceptable. It limits the magnitude of supply room temperature difference and/or space cooling loads for a given supply airflow rate.

A number of design parameters must be considered, including supply airflow rate, supply air temperature, air temperature at floor level, vertical temperature gradient, maximum air velocity at floor level, stratification height, contaminant concentration gradient, energy consumption, first cost, and maintenance. It is prudent to be cautious and clarify assumptions when applying recommendations from design guides.

DV pros and cons

The unique characteristics of large underground institutional facilities can be addressed by applying technologies that are merging into common practice. Displacement ventilation (DV) offers many advantages, including improved indoor air quality, low acoustics, and energy efficiency. It is now being applied in large open spaces as well as offices and classrooms. If a building’s design accommodates the special requirements of DV, it can be a healthy option and energy-efficient opportunity.

Advantages

DV provides better acoustics and better air quality than mixed-flow systems. Mixed-flow systems tend to be louder because of the higher velocity required from diffusers. Diffuser noise can be difficult to attenuate. Applying DV diffusers rather than mixed-flow diffusers can reduce sound levels by an NC factor of 5.

Lower supply velocity at diffusers means lower pressure drop, smaller fans, and less energy consumption. Fan horsepower reductions can be attributed to less air movement.

DV can use fewer diffusers and less ductwork.

DV introduces supply air at the playing/teaching floor, improving indoor air quality by reducing accumulation of CO2, odor, and indoor contaminants.

DV has a higher ventilation effectiveness than mixed-flow systems.

Free cooling may be available most of the year.

If 100% outdoor air and exhaust is used, the heat gain due to the lights and roof can be eliminated from building cooling loads.

Disadvantages

DV cannot be applied as widely as mixed-air systems.

DV can add complexity to supply air ducting.

DV diffusers are more expensive than mixed-flow diffusers.

The room neutral temperature for a DV system is higher than that of a conventional mixing system.

Schultz is an associate vice president at Cannon Design. His expertise includes all facets of mechanical systems, from heating and cooling plants to distribution and terminal units. He most recently served as mechanical engineer for Kaleida Health’s Gates Vascular Institute at the State University of New York at Buffalo.

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